Paint for pre-coated metal (PCM) requires excellent weather resistance, corrosion resistance, scratch resistance, gloss, hardness, and flexibility, and has been variously used in exterior decoration materials for construction and household appliances. However, pre-coated metal must have specifications that may satisfy both high strength and excellent flexibility. With respect to a paint composition, strength and flexibility are properties contradictory to each other. A coating layer having high strength has excellent scratch resistance, but has low chipping resistance due to high brittleness and is very sensitive to heat. In contrast, a coating layer having high flexibility has excellent chipping resistance and is less sensitive to heat, but has low scratch resistance. Also, a coating layer of pre-coated metal may be greatly damaged by a welding process.
Recently, development of a technique for manufacturing a highly formable color steel sheet for an automobile has been required for automobile manufacturers for the purposes of manufacturing cost reduction, development of an environmental-friendly process, and simplification of processing. A method of assembling a full vehicle by using pre-coated metal is an innovative technique in terms of environmental friendliness and productivity, because all wet painting processes, such as pretreatment, electrodeposition, intercoating, and top coating processes, which are indispensible in a typical automotive coating process, may be omitted.
However, cracks in a coating layer may occur during curved surface forming due to differences in physical properties of a steel sheet and a molding material of the coating layer and appearance characteristics may be deteriorated. As a result, the method of assembling a full vehicle by using pre-coated metal has not been used so far in an automotive industry requiring attractive appearance and high gloss.